Add doctype to the index.html we provide

I think we should do this. Would this go here? 
https://github.com/jquery/themeroller.jquerymobile.com/blob/master/inc/index.html.inc

We have inconsistencies also between <!doctype html> and <!DOCTYPE html> in places... which one should we go for? (I have a preference for <!doctype html>)
cc: @scottgonzalez @arschmitz @jaspermdegroot @cgack @gabrielschulhof 
